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SubjectRe: [PATCH tip/core/rcu 30/30] rcu: Make rcu_barrier() account for offline no-CBs CPUs
Hi Paul,

On Fri, Feb 14, 2020 at 03:56:07PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
>
> Currently, rcu_barrier() ignores offline CPUs, However, it is possible
> for an offline no-CBs CPU to have callbacks queued, and rcu_barrier()
> must wait for those callbacks. This commit therefore makes rcu_barrier()
> directly invoke the rcu_barrier_func() with interrupts disabled for such
> CPUs. This requires passing the CPU number into this function so that
> it can entrain the rcu_barrier() callback onto the correct CPU's callback
> list, given that the code must instead execute on the current CPU.
>
> While in the area, this commit fixes a bug where the first CPU's callback
> might have been invoked before rcu_segcblist_entrain() returned, which
> would also result in an early wakeup.
>
> Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: <stable@vger.kernel.org> # 5.5.x
> ---
> include/trace/events/rcu.h | 1 +
> kernel/rcu/tree.c | 32 ++++++++++++++++++++------------
> 2 files changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 5e49b06..d56d54c 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -712,6 +712,7 @@ TRACE_EVENT_RCU(rcu_torture_read,
> * "Begin": rcu_barrier() started.
> * "EarlyExit": rcu_barrier() piggybacked, thus early exit.
> * "Inc1": rcu_barrier() piggyback check counter incremented.
> + * "OfflineNoCBQ": rcu_barrier() found offline no-CBs CPU with callbacks.
> * "OnlineQ": rcu_barrier() found online CPU with callbacks.
> * "OnlineNQ": rcu_barrier() found online CPU, no callbacks.
> * "IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index d15041f..160643e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3098,9 +3098,10 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> /*
> * Called with preemption disabled, and from cross-cpu IRQ context.
> */
> -static void rcu_barrier_func(void *unused)
> +static void rcu_barrier_func(void *cpu_in)
> {
> - struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> + uintptr_t cpu = (uintptr_t)cpu_in;
> + struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>
> rcu_barrier_trace(TPS("IRQ"), -1, rcu_state.barrier_sequence);
> rdp->barrier_head.func = rcu_barrier_callback;
> @@ -3127,7 +3128,7 @@ static void rcu_barrier_func(void *unused)
> */
> void rcu_barrier(void)
> {
> - int cpu;
> + uintptr_t cpu;
> struct rcu_data *rdp;
> unsigned long s = rcu_seq_snap(&rcu_state.barrier_sequence);
>
> @@ -3150,13 +3151,14 @@ void rcu_barrier(void)
> rcu_barrier_trace(TPS("Inc1"), -1, rcu_state.barrier_sequence);
>
> /*
> - * Initialize the count to one rather than to zero in order to
> - * avoid a too-soon return to zero in case of a short grace period
> - * (or preemption of this task). Exclude CPU-hotplug operations
> - * to ensure that no offline CPU has callbacks queued.
> + * Initialize the count to two rather than to zero in order
> + * to avoid a too-soon return to zero in case of an immediate
> + * invocation of the just-enqueued callback (or preemption of
> + * this task). Exclude CPU-hotplug operations to ensure that no
> + * offline non-offloaded CPU has callbacks queued.
> */
> init_completion(&rcu_state.barrier_completion);
> - atomic_set(&rcu_state.barrier_cpu_count, 1);
> + atomic_set(&rcu_state.barrier_cpu_count, 2);
> get_online_cpus();
>
> /*
> @@ -3166,13 +3168,19 @@ void rcu_barrier(void)
> */
> for_each_possible_cpu(cpu) {
> rdp = per_cpu_ptr(&rcu_data, cpu);
> - if (!cpu_online(cpu) &&
> + if (cpu_is_offline(cpu) &&
> !rcu_segcblist_is_offloaded(&rdp->cblist))
> continue;
> - if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> + if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
> rcu_barrier_trace(TPS("OnlineQ"), cpu,
> rcu_state.barrier_sequence);
> - smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> + smp_call_function_single(cpu, rcu_barrier_func, (void *)cpu, 1);
> + } else if (cpu_is_offline(cpu)) {

I wonder whether this should be:

else if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_is_offline(cpu))

? Because I think we only want to queue the barrier call back if there
are callbacks for a particular CPU. Am I missing something subtle?

Regards,
Boqun

> + rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
> + rcu_state.barrier_sequence);
> + local_irq_disable();
> + rcu_barrier_func((void *)cpu);
> + local_irq_enable();
> } else {
> rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> rcu_state.barrier_sequence);
> @@ -3184,7 +3192,7 @@ void rcu_barrier(void)
> * Now that we have an rcu_barrier_callback() callback on each
> * CPU, and thus each counted, remove the initial count.
> */
> - if (atomic_dec_and_test(&rcu_state.barrier_cpu_count))
> + if (atomic_sub_and_test(2, &rcu_state.barrier_cpu_count))
> complete(&rcu_state.barrier_completion);
>
> /* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> --
> 2.9.5
>

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